Practical knowledge

User issue: There is an overshoot and overshoot phenomenon when using MFC

At the beginning of ventilation, the flow rate will instantly reach the maximum measurable value and display an alarm message, and then quickly stabilize to the set value. The customer feels that the YIDU MFC has a serious overshoot problem. After detailed communication with the customer, it was found that the customer first set the flow rate of MFC and then opened the shut-off valve to ventilate the pipeline. Should MFC be ventilated or flow rate set first during use?

Common MFC connection methods

working principle

MFC measures gas flow through the flow measurement section and sends feedback signals to adjust the valve opening size based on the measured values, achieving the goal of controlling flow. When the measured flow rate is less than the set flow rate, the valve opening will be increased to increase the gas flow rate, and vice versa.

      MFC working schematic diagram

Once you understand the working principle of MFC, it becomes easier to comprehend the problems encountered by customers

In the absence of ventilation, after setting the flow rate of MFC, the flow sensor detects that the flow rate is zero, which is less than the set value. Therefore, the microprocessor sends a signal to open the valve and check the flow rate again. It is found that the flow rate is still zero, which is still less than the set value. The microprocessor can only continue to issue control signals until the valve is fully opened and maintained in this state.

At this point, the shut-off valve is opened and the gas instantly begins to flow. When the valve is opened to its maximum, the gas will flow through the MFC at a large flow rate. MFC detected that the flow rate exceeded the set value, so it controlled the valve opening to decrease and stabilize the actual flow rate at the set value. This is the actual working process of MFC when setting the flow rate before ventilation.

Answer Conclusion

This is not an issue with MFC itself, but rather a problem with the sequence of operations when using MFC. The correct sequence of operation is to ventilate first, and then set the flow rate.